fix(ui): scroll-coupled backdrop pan that works in every browser

CSS animation-timeline: scroll() is Chromium-only, so the backdrop didn't move
for non-Chromium browsers. Replace it with a tiny rAF-throttled scroll handler
in site.js (the existing same-origin script — CSP stays strict, no inline JS)
that drives a --bg-shift CSS var off scroll position. The board now pans
vertically in lock-step with scroll and sits still at rest, in all browsers.

public/site.js

@@ -55,5 +55,37 @@
         io.observe(el);
       });
     }
+
+    // --- Scroll-coupled background pan -------------------------------------
+    // The Bat-Computer backdrop pans vertically in lock-step with scroll —
+    // moving ONLY while you scroll, still at rest. Plain scroll listener so it
+    // works in every browser (unlike CSS animation-timeline: scroll(), which is
+    // Chromium-only). rAF-throttled; sets a CSS var the transform reads.
+    var bgImgs = document.querySelectorAll(".site-bg__img");
+    if (bgImgs.length) {
+      var ticking = false;
+      function updateBg() {
+        ticking = false;
+        var doc = document.documentElement;
+        var max = doc.scrollHeight - window.innerHeight;
+        var p = max > 0 ? Math.min(1, Math.max(0, window.scrollY / max)) : 0;
+        // +half a range at the top → -half at the bottom: scrolling down pans
+        // the camera down the board. Range kept within the image's overhang so
+        // an edge never shows (image is 215% tall → 57.5vh spare each side).
+        var shift = (0.5 - p) * window.innerHeight * 0.7;
+        for (var i = 0; i < bgImgs.length; i++) {
+          bgImgs[i].style.setProperty("--bg-shift", shift.toFixed(1) + "px");
+        }
+      }
+      function onScroll() {
+        if (!ticking) {
+          ticking = true;
+          window.requestAnimationFrame(updateBg);
+        }
+      }
+      window.addEventListener("scroll", onScroll, { passive: true });
+      window.addEventListener("resize", onScroll, { passive: true });
+      updateBg();
+    }
   });
 })();